Measurement of thermal diffusivity in polymer melts using forced Rayleigh light scattering

Abstract

Thermal diffusivity measurements on three polymer melts were made using the Forced Rayleigh Light Scattering technique. The polymers, which were tested at room temperature where they are in the molten state, included a polydimethylsiloxane and two polyisobutylenes. The optical setup and procedures developed in this study to conduct thermal Forced Rayleigh Light Scattering experiments are shown to be capable of producing thermal diffusivity data with a high degree of accuracy and precision. From measurements on a reference fluid (ethanol), experimental error was estimated to be no greater than 2%, and could be reduced to less than 1% by appropriate design of a series of experiments. Discrepancies of 4 and 14% in thermal diffusivity data on the polymer samples between measured values and those found in the literature were observed. It is suggested that these deviations are attributable to either sample variations or to errors in the techniques used in previous investigations. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1069–1078, 1999

Measurements of Flow-Induced Anisotropic Thermal Conduction in a Polyisobutylene Melt Following Step Shear Flow

“…The Levenberg-Marquardt method used to fit eq 8 to V data for t > t p and determine τg is described elsewhere. 27 The inset to Figure 4 shows that the residuals have a Gaussian distribution. Consistency of the measured photodetector voltage with the biexponential function in eq 8 was carefully examined and interpreted as validation of the assumed thermophysical model.…”

“…Because the grating period Λ is much smaller than the spot size of the writing laser (w ) 1 mm), the dynamics of the grating temperature field can be decoupled from the bulk temperature in the sample. Furthermore, if conditions for the plane grating approximation 25,27 are satisfied, δT following a pulse of the writing laser of duration tp is governed by where F is the density and Cp is the specific heat. The solution of eq 5 can be written as where the grating relaxation time τg is given by where D ) k/FCp is the thermal diffusivity tensor.…”

“…The high correlation of the data with the expected linear relation is strong evidence that the assumed thermophysical model underlying the FRS technique is valid. 23,27 The slope of the solid line through the symbols in Figure 5 gives the thermal diffusivity at an angle of π/4 from the flow direction. As expected, the slope of this line deviates from the equilibrium value shown by the dashed line in Figure 5.…”

“…For each grating size Λ, sample absorption coefficient K and writing laser pulse time t p were optimized to maximize the signal-to-noise ratio for V while restricting bulk heating (<0.1 K). 21,27 The equilibrium value of thermal diffusivity for the PIB131K measured using FRS was found to be Deq ) (6.65 ∆n ) (n 11 -n 22 ) cos 2 (R) + 2n 12 sin(R) cos(R) + (n 22 -n 33 )…”

“…This value is consistent with the value found for the broad molecular weight distribution polyisobutylene (PIB85K) considered in a previous study. 21,27 Following a step strain, the prisms were aligned creating different optical paths for the writing and reading laser beams. This allowed the grating vector to take four different orientations with respect to the coordinate system used to define the flow so that different projections of the thermal diffusivity tensor could be sampled with the FRS technique.…”